The following abbreviations used in the specification and/or the drawings are defined as follows:    3GPP third generation partnership project    BSS base station system    CSG closed subscriber group    DL downlink (network towards UE)    eNodeB base station of a LTE/LTE-A system    EPLMN equivalent PLMN    E-UTRAN evolved UTRAN    GERAN GSM/edge radio access network    GSM global system for mobile communications    ID identifier    LAU location area update    LTE long term evolution (of the UTRAN system)    MME mobility management entity    MSC mobile switching center    NAS network access stratum    PLMN public land mobile network    S-GW serving gateway    RAT radio access technology    RR radio resource    UE user equipment    UL uplink (UE towards network)    UMTS universal mobile telecommunication system    UTRAN universal terrestrial radio access network
In the E-UTRAN system relevant to these teachings there are conventional network access nodes/cells termed eNodeBs which serve all users, and also subscriber group CSG cells such as home eNodeBs which are available for traffic (voice and/or data) to only those subscribers registered with the CSG and possibly also certain allowed guests. Any given CSG may include a group of cells (such as a corporate or university campus) or a single cell. CSGs may allow traffic access for non-subscribers for emergency calls but these teachings relate to routine, non-emergency access.
These different types of cells or access nodes may be, termed more generally as public access nodes/public cells and private access nodes/private cells. Other wireless systems (GERAN, GSM, UTRAN, WCDMA) have either implemented or are considering implementing similar such private cells as more functionality is shifted from higher in the radio access network toward the base stations/access nodes. In these systems the UE is to keep a list, termed a CSG whitelist or CSG allowed list, which recites all the identities of the CSG cells for which that particular UE has access rights. Other radio access technologies RATs use similar whitelists which may be known by different terms. For LTE and UMTS the CSG whitelist is a combination of the UE's allowed CSG list and the operator's CSG list (see for example 3GPP TS 23-122 v10.4.0 for further details).
Many mobile user radios are now capable of operating in multiple RATs, and the examples below are in the non-limiting context of a UE handing over between GERAN cells and checking if a nearby UTRAN/LTE CSG cell is allowed. In recent 3GPP discussions related to the handling of the CSG whitelists when the UE performs its access check (where the UE checks if a given CSG is allowed or not) prior to reporting of the CSG cell resulted in agreement that the UE should take into account the list of “equivalent PLMNs” in addition to the registered and selected PLMN (see for example document SP-110415 entitled LS ON EQUIVALENT PLMN [TSG SA Meeting #51; Bratislava, Slovakia; 6-8 Jun. 2011]. The CSG whitelist contains [CSG ID, PLMN ID] pairs indicating which CSGs in which PLMNs the mobile station is allowed to access. Hence a CSG cell is allowed for a UE if, in addition to the cell's CSG ID being in the whitelist, its associated PLMN ID is determined as a valid one by the UE, valid being equal to the PLMN ID of the selected PLMN or the registered PLMN or one of the PLMNs in its EPLMN list. In dedicated mode, the registered and selected PLMNs are the same.
3GPP TS 24.008 v10.3.0 (2011 June) provides that the UE will locally store a list of EPLMNs received from the network in which it is registered. This list is replaced or deleted at the end of each location/registration update procedure, routing area update procedure and GPRS attach procedure. The stored list consists of a list of equivalent PLMNs as downloaded by the network plus the PLMN code of the registered PLMN that downloaded the list. A problem arises in that when handing over between cells, the UE will not always have the correct PLMN/EPLMN and so its view of what nearby CSG cells are allowed is not always accurate.
FIG. 1 illustrates a relevant wireless environment: a UE 20 is under control of a GERAN cell-A 24 which is in PLMN1 and is handing over to another GERAN cell-B 28 in PLMN2. Nearby there is a CSG cell-C 30 with PLMN-IDa. Cell-A 24 is the UE's serving cell and so the UE obtains from cell-A 24 its (first) whitelist 101, which the cell-A 24 gets from its MSC-A 22 and which includes PLMN1 and list EPLMN1. Along with the CSG IDs, this is the whitelist the UE 20 has stored in its memory. The valid PLMNs for the UE 20 in cell-B 28 are in its (second) whitelist, which cell-B 28 obtains from its MSC-B 26 and which includes PLMN2 and EPLMN2. For this UE 20, CSG cell-C 30 with PLMN-IDa may not be an allowed cell in cell-A 24 (if PLMN-IDa is not within the set {PLMN1, EPLMN1}) but it could be an allowed in cell-B 28 (if PLMN-IDa is within the set {PLMN2, EPLMN2}) or vice versa. CSG cell-C 30 is a potential target cell for handover and ideally the mobile station 20 should determine prior to reporting the CSG cell-C whether it is an allowed cell.
In the GERAN dedicated mode upon handover to a cell of a different location area, the location area update (LAU) procedure can only be performed after the dedicated connection is released. While a LAU may only be triggered by the UE, unlike the network the UE is not necessarily aware of the change of location area at handover. Thus it is not aware that there may be a change to the PLMN or EPLMN lists. This had not been an issue previously because only recently has mobility to CSG cells been considered.
Mobility to CSG cells in the connected/dedicated mode relies on the ability of the UE to report CSG cells and on the ability of the network to hand the UE over to a previously reported CSG cell. If a UE 20 in the connected/dedicated mode needs to report a UTRAN CSG cell-C 30 as in FIG. 1 which is in cell-B 28, then the CSG access check performed by the UE 20 for the CSG cell-C 30 identified as (CSG-IDa, PLMN-IDa) may not yield the correct allowed/not allowed decision because the UE 20 does not necessarily have the correct whitelist set {PLMN2, EPLMN2} that is associated with that cell-B 28.
One approach to resolve this issue is noted at document S2-112640, entitled INTERPLMN HANDOVER TO A CSG CELL by Samsung, Nokia Siemens Networks and Huawei [3GPP TSG SA WG2 Meeting #85; Xian, China; 16-20 May 2011]. This document is a change request for 3GPP TS 23.401, and specifically it sets forth that the source network validate the CSG membership of the UE in the target CSG cell using the CSG-ID list of the serving PLMN-ID before allowing the handover. As will be detailed below, this is seen to be only a partial solution for it fails to address the problem in which, due to the UE 20 not having the valid list of EPLMNs, the nearby CSG cell is not even reported by the UE when in fact that nearby cell is an allowed CSG cell.
Embodiments of these teachings detailed below resolve the above problems in correctly identifying handover CSG cells across different PLMNs.